1. Field of the Invention
The present invention concerns a process for the oxidation of cumene to cumene hydroperoxide, wherein the selectivity has been improved.
2. Description of Related Art
Phenol is commonly manufactured through a cumene procedure, wherein cumene is oxidized to cumene hydroperoxide (CHP) and the resulting oxidation product mixture is concentrated and subjected to a cleavage reaction. Subsequently, the cleavage product mixture is conducted to a distillation section, wherein the main products of the cleavage reaction, i.e. phenol and acetone, are first separated and then purified through a series of distillation steps or other purification steps.
In the prior art, oxidation of cumene is generally carried out using a so-called wet-oxidation procedure, in which oxidation takes place in solution with the help of an aqueous solution of, for example, a carbonate. Dry oxidation procedures, where the only compounds introduced into the reaction mixture are the starting material (cumene) and the oxidation gas, are getting more common.
A disadvantage of the wet procedures is that they require, among others, a step of removing the carbonate and neutralizing the aqueous oxidized mixture, which has been rendered alkaline by the carbonate, before the oxidation product (CHP) can be concentrated.
The liquid phase oxidation of cumene is explained in terms of a radical mechanism by Kazua Hattori et al. in Journal of Chemical Engineering of Japan, vol. 3, no. 1, (1970), p. 72-78. The main side products formed in the oxidation are acetophenone and carbinol. The process is generally thought to follow the following scheme

The formation of acetophenone (AcPh) is problematic, since it is not separated from the product mixture downstream from the oxidation. Carbinol (particularly dimethyl benzyl alcohol, DMBA) is partly recovered by converting it to α-methyl styrene (AMS) and by the subsequent hydrogenation of AMS to cumene. However, AMS as such is a source of heavy products, such as AMS dimers, which are not recovered downstream.
Cumene hydroperoxide selectivity is normally calculated on a molar basis from the cumene oxidation products:CHP/(CHP+AcPh+DMBA+2DCP)(DCP=dicumyl peroxide.) Typical values for the total selectivity in the oxidation are in the range of 92-94%.
Operation and design parameters of the oxidation, such as the pressure, the temperature, the CHP concentration, the residence time, the number of reactors, the treatment of the recycle streams, the treatment of the off-gas and cooling of the reactors, have an effect on the selectivity. Thus, the right selection of these parameters is important. It is also important that the feeds of cumene and, for example, air are properly treated to remove inhibitors, such as phenol, AMS, sulphur and carbon dioxide, or other impurities, such as inorganic acids or bases or free-radical generating compounds, since these impurities may cause the premature decomposition of the newly formed CHP. This premature decomposition may for example be caused by the impurities lowering the temperature at which the CHP decomposes. The presence of these impurities may also lead to a different, undesirable decomposition mechanism, thus leading to the formation of other impurities.
Oxidation of cumene into cumene hydroperoxide (CHP) has been thoroughly described in the prior art (as in GB 1006319, JP 4305564, JP 2000290249, JP 2000302752 and JP 2003231674), but there is still a need for further improving the process, since every change in a process parameter may have a significant effect on the others, thus causing a significant change for the product quality and quantity. For example, a decreased reaction rate may be compensated by an increase in the temperature, whereas a higher temperature causes an increase in the decomposition of CHP. Further, the CHP decomposition product, phenol, will cause a decrease in the reaction rate even in small concentrations, such as from a level of 10 ppm, e.g. a level of 10-100 ppm. Formation of acetophenone, on the other hand, will cause a decrease in the pH and an increase in the decomposition of phenol.
Improvements have been attempted in the prior art, for example, by positioning the oxidation reactors at reducing elevations, as in JP 2000290249, whereby the need for pumps or other similar means for moving the oxidation reaction mixture from one reactor to the next is removed, or by making the capacity of the oxidation reactors smaller one by one, as in JP 2000302752, whereby the reaction rate will be highest in the first reactor. In JP 2003231674, it has been attempted to optimize the oxidation reaction by limiting the velocity of the oxygen-containing gas bubbled through an oxidation reactor.